Plasma-assisted chemical reactions, such as plasma-enhanced chemical vapor deposition ("PECVD"), have been widely used for film deposition in the semiconductor and flat panel display industries in the manufacture of thin film transistors ("TFT"s) for active-matrix liquid crystal displays ("AMLCD"s), e.g. In accordance with PECVD, a substrate is placed in a vacuum deposition chamber that is equipped with a pair of parallel plate electrodes. One of the electrodes, e.g., the lower electrode, generally referred to as a susceptor, holds the substrate. The other electrode, e.g., the upper electrode, functions as a gas inlet manifold or shower head. During deposition, a reactant gas flows into the chamber through the upper electrode and a radio frequency (RF) voltage is applied between the electrodes to produce a plasma within the reactant gas. The plasma causes the reactant gas to decompose and deposit a layer of material onto the surface of the substrate.
Two types of materials often deposited are p-Si and a-Si. Both a-Si and p-Si are used to fabricate TFTs, such as those used in AMLCDs. Other applications include solar cells, etc. Typically, p-Si is deposited from a silane source at a substrate temperature above about 600 degrees Celsius (.degree. C.). If the substrate temperature is lower, the initially deposited structure is a-Si; i.e., it has no definite arrangement of atoms.
Thus, it is often easier to deposit a-Si because less substrate heating is required. However, devices made with a-Si often exhibit poorer performance when compared to the same devices made with p-Si. E.g., field effect mobility, which is generally accepted to be one of the most important device characteristics, may be better for p-Si than for a-Si by orders-of-magnitude. Thus, using p-Si films instead of a-Si films may increase the performance of devices such as flat panel displays. Fortunately, a p-Si film may be obtained from an a-Si film by annealing the a-Si film.
Annealing, however, may be difficult in some cases. One reason for this concerns the quality of the substrate used. For a commercially-viable application of flat panel displays, e.g., the glass substrates used should be inexpensive. However, the maximum processing and deposition temperature of an inexpensive glass substrate may be relatively low, e.g., below 400.degree. C. At these temperatures, only a-Si can be deposited.
Current annealing methods encounter difficulties with such substrates. These techniques heat the substrate and the film at the same time, and thus are problematic for the reason noted above.
Conventional methods of performing annealing include, among others, laser annealing, thermal annealing, and lamp annealing. These approaches have certain inherent drawbacks. Laser annealing requires a complicated scanning laser configuration. Thermal annealing requires, e.g., a source of hot gas in addition to the gas sources already required for deposition. Lamp annealing requires a complicated multiple-lamp system to ensure temperature uniformity.